The present invention is directed to blast cleaning with abrasives. More specifically, the present invention is directed to an improved apparatus for blast cleaning with at least two different types of abrasives.
In order to clean a solid surface to preserve metal against deterioration, remove graffiti from stone or simply to degrease or remove dirt or other coatings from a solid surface, it has become common practice to use an abrasive blasting technique wherein abrasive particles are propelled by a fluid against the solid surface in order to dislodge the previously applied coatings, scale, dirt, grease or other contaminants. Such abrasive blasting has been used favorably, for example, to degrease metal and is increasingly replacing the environmentally hazardous solvent cleaning treatments.
Standard sand blasting equipment consists of a pressure vessel or blasting pot to hold particles of a blasting medium such as sand, connected to a source of compressed air by means of a hose and having a means of metering the blasting medium from the blast pot, which operates at a pressure that is the same or slightly higher than the conveying hose pressure. The sand/compressed air mixture is transported to a nozzle where the sand particles are accelerated and directed toward a workpiece. Flow rates of the sand or other blast media are determined by the size of the equipment. Commercially available sand blasting apparatus typically employ media flow rates of 20-30 lbs/min. About 1.2 lbs of sand are used typically with about 1.0 lb of air, thus yielding a ratio of 1.20.
When it is required to remove coatings such as paint or to clean relatively soft surfaces such as aluminum, magnesium, plastic composites and the like, less aggressive abrasives, including inorganic salts such as sodium bicarbonate and sodium chloride, can be used in conventional sand blasting equipment. The medium flow rates required for less aggressive abrasives are substantially less than that used for sand blasting, and have been determined to be from about 0.5 to about 10.0 lbs/min., using similar equipment. This requires a much lower medium to air ratio, in the range of about 0.05 to 0.25.
However, difficulties are encountered in maintaining continuous flow at these low flow rates when conventional sand blasting equipment is employed. Fine particles of a medium such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, they tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. Flow aids such as hydrophobic silica have been added to the bicarbonate in an effort to improve the flow, but maintaining a substantially uniform flow of bicarbonate material to the nozzle has been difficult to achieve. Sporadic flow of the blasting media leads to erratic performance which in turn results in increased cleaning time and even to damage of somewhat delicate surfaces.
Commonly assigned U.S. Pat. Nos. 5,081,799 and 5,083,402 disclose a modification of conventional blasting apparatus for directing the less aggressive abrasives to a substrate surface by providing a separate source of line air to a closed supply pot through a pressure regulator to provide a greater pressure in the supply pot than is provided to the conveying hose. This differential pressure is maintained by an orifice having a predetermined area and situated between the supply pot and the conveying hose. The orifice provides an exit for the blast media and a relatively small quantity of air from the supply pot to the conveying hose, and ultimately to the nozzle and finally to a workpiece. The differential air pressure, typically operating between 1.0 and 5.0 psi with an orifice having an appropriate area, provides accurate control over media flow rates. An example of such a system is the Accustrip System.TM. developed by Church & Dwight of Princeton, N.J. The blasting apparatus disclosed in U.S. Pat. Nos. 5,081,799 and 5,083,402 are a significant improvement over standard sand blasting equipment, especially for blasting with less aggressive abrasive media.
Suitable abrasive materials which can be used to clean solid surfaces with a blasting apparatus such as the Accustrip System.TM. include, but are not limited to, such materials as sodium bicarbonate, sodium chloride, aluminum oxide, magnesium oxide, sand, and the like as well as mixtures thereof. Any type of abrasive having a Mohs hardness of up to about 10 can be blasted from such apparatus. Other cleaning adjuvants which can be mixed with abrasives or separately blasted at a workpiece include, but are not limited to, such materials as surfactants, sanitizing agents, and corrosion inhibitors in solid form or in liquid form applied separate from the abrasive as in a pressurized liquid stream or sprayed onto the abrasive particles or inert carriers.
Blasting apparatus such as the Accustrip System.TM. are a very effective blasting apparatus for cleaning all solid surfaces including relatively soft solid surfaces such as aluminum, magnesium, plastic composites, and the like. Such blasting apparatus as the Accustrip System.TM. can blast clean with one type of abrasive or a combination of abrasives and/or adjuvants at the same time. However, if such systems blast clean with more than one type of abrasive, the abrasives and, if used, other cleaning adjuvants are premixed in a desired proportion then prepacked before arriving at the blasting site, or are premixed at the site before placing the abrasive mixture in the blasting pot of the apparatus. Abrasive mixtures having the desired proportions of each abrasive or adjuvant are expelled from a nozzle orifice toward a workpiece to be cleaned.
Often the nature of the substrate being blast cleaned can vary drastically from area to area. Thus, there is a need for using different types of abrasives or cleaning adjuvants. While a premixed abrasive can be beneficial in tailoring the blast cleaning operation to improve the removal of specific contaminants which may predominate on the substrate being cleaned, it still would be worthwhile to provide the operator more flexibility during the blasting process to choose which particular abrasive and/or cleaning adjuvant to use to increase blasting productivity and as will conserve materials to improve overall processing efficiency.
U.S. Pat. No. 5,334,019 discloses a dental apparatus for cleaning teeth employing two hoppers for introducing abrasive material into a flexible hose with a gas flow stream. Different sized and/or types of particles can be contained within each separate hopper. A control system is employed to introduce abrasive material from either or both hoppers into the gas flow stream. Such a system is specifically employed for removing and/or cutting tooth structures of enamel and dentin and for removing amalgam, composites, other dental tooth filling materials and/or stain.
Each hopper of the dental apparatus is connected by feed valves to a mixing chamber which interrupts media flow from each hopper to the gas stream in the flexible hose. It is in the mixing chamber where abrasive materials are mixed prior to passing to the gas stream in the flexible hose. Alternatively, the mixing chamber can be eliminated and lines from each hopper are joined at a Y junction where abrasive materials from each hopper can be introduced to a gas stream directly. The Y junction has a flapper valve which allows an operator to alternate from one abrasive material to another. A disadvantage of such an apparatus having added mechanical components in flow lines is a potential for residual build up of abrasive material at junctions where flow lines and mechanical components meet resulting in blockage at the junctions, especially if such apparatus are not properly maintained. Moreover, mechanical components wear out with continued use and must be replaced. Thus, the more mechanical components an apparatus has the costlier it is to maintain and less desirable for a relevant industry to employ.
Although there are effective blasting apparatus for cleaning solid surfaces, a primary object of the present invention is to provide for an improved apparatus and method for blast cleaning a solid surface with at least two different abrasives and/or adjuvants and which allows for continued and immediate changes in the proportions of the individual abrasives directed from the blast nozzle, and, thus, provide an operator with greater flexibility during the blast cleaning operation.
Another object is to provide for a blasting apparatus which provides for blast cleaning with at least two different abrasives and/or adjuvants, which provides for precise control over the relative proportions of the separate media directed from the nozzle and which maintains a continuous flow rate of each blasting media to the nozzle.
A further object is to provide a blasting apparatus for blast cleaning with at least two different abrasives and/or cleaning adjuvants contained in separate blasting pots where each blasting pot of the apparatus can be operated independently of each other.
Still yet another object of the present invention is to blast clean a solid surface with at least two abrasives having a different Mohs hardness and/or particle size where the flow rate of the individual abrasive can be independently controlled.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention.